Science 359, 892–896 (2018)

Metasurfaces with strongly anisotropic optical properties can support deep
subwavelength-scale confined electromagnetic waves (polaritons), which promise
opportunities for controlling light in photonic and optoelectronic applications. We
developed a mid-infrared hyperbolic metasurface by nanostructuring a thin layer of
hexagonal boron nitride that supports deep subwavelength-scale phonon polaritons
that propagate with in-plane hyperbolic dispersion. By applying an infrared nanoimaging
technique, we visualize the concave (anomalous) wavefronts of a diverging polariton
beam, which represent a landmark feature of hyperbolic polaritons. The results illustrate
how near-field microscopy can be applied to reveal the exotic wavefronts of polaritons
in anisotropic materials and demonstrate that nanostructured van der Waals materials
can form a highly variable and compact platform for hyperbolic infrared metasurface
devices and circuits.